PROJECT SUMMARY A comprehensive understanding of bacterial pathogenesis not only requires a detailed knowledge of the genome and proteome, but also the peptidome elaborated during the progression of infection. The overall objective of this proposal is to build upon our exciting preliminary observations describing the identification of endogenous microproteins and peptides detected in cell-free supernatants of community-associated (CA) MRSA cultures. Within this group we identified two novel microproteins originating from an unannotated locus in the CA-MRSA TCH1516 genome. We found that these microproteins, termed S. aureus microprotein 1 (SAM1) and S. aureus microprotein 2 (SAM2), are highly conserved among Staphylococci and are regulated by the classical accessory gene regulatory system. We have started to characterize these factors, showing that SAM1 appears to act as a canonical cytolysin. Intriguingly, SAM2 possesses unique bioactivity, the perturbation of keratin networks that promotes an in vivo switch from a localized S. aureus skin infection to an invasive dissemination to the underlying tissues. The central hypothesis of this proposal is that SAMs significantly contribute to CA-MRSA?s ability to cause disease in a host. In Aim 1, we will dissect the pathogenic contributions of SAM1 as a functional cytolysin in vitro and in vivo. Given the bioactivity of SAM1 is inhibited by serum lipoproteins, we will focus on its role once CA-MRSA is in an intracellular environment. Our strong preliminary data suggests SAM1 selectively binds to prohibitins (PHBs) across differential host cell types. Thus, we will take the study of CA-MRSA cytolysins in a new direction by dissecting the potential role of PHBs as targets of SAM1. In Aim 2, we will perform a structure and function analysis of the interaction between SAM2 and keratin. Our data shows that this interaction has an important in vivo consequence during the shift from a local to invasive infection type. By a powerful combination of microbiology and multi-omic approaches, we will define the SAM2 structural contact points required for pathogenesis and will detail host pathways that are impacted by SAM2 during the invasive switch. In Aim 3, to assess the broader significance of our discovery, we will determine the pathogenic roles SAMs play in a diverse library of CA-MRSA strains, in addition to other Staphylococcal pathogens that express bioactive SAM homologs. Aim 3 is critical given that the importance of SAMs in other Staphylococci beyond CA-MRSA TCH1516 remains unknown. This proposal is highly innovative because it departs from the current focus of investigating host-pathogen interactions through the more established genomic and proteomic workflows. The proposed work is highly significant given it can drive the development of anti-Staphylococcal therapies based on a relatively new and largely mysterious molecular paradigm - the peptidome.